However, there were conflicting claims that he may have taken his own life after suffering from severe depression.

Friends raised the possibility of suicide and said Berezovsky had been “destroyed” by losing a £3 billion legal action with his former business partner, Roman Abramovich, the owner of Chelsea football club. Berezovsky had been living in exile in Britain since 2000.

He was given asylum in 2003, which created tensions between London and Moscow.

Once worth as much as £3 billion, he had suffered financial and personal problems, his friends said.

News of Berezovsky’s death emerged in a posting on Facebook by his son-in-law, Egor Schuppe. Mr Schuppe simply posted: “Boris Berezovsky is dead.”

Police stand guard at a roadblock near where it is believed Boris Berezovsky lived (Getty Images)

Berezovsky, an outspoken critic of Vladimir Putin, the Russian president, had long feared for his life and had survived repeated assassination attempts — although none in recent years. He had been due to be a witness at the inquest next month into the death of Alexander Litvinenko, the former KGB spy who was murdered in 2006 after being poisoned by radioactive polonium-210 in what is believed to have been a Kremlin-sanctioned assassination.

The police investigation into Berezvosky’s death is the first of its kind since the murder of the spy. The scale of the investigation was announced at 1am this morning by police in a reflection of its seriousness.

Mr Putin’s spokesman said the Russian president had been informed of Berezovsky’s death. The spokesman claimed that in recent weeks the oligarch had written a letter to Mr Putin “asking for forgiveness” and seeking “help to return to the motherland”.

Such a move would have been a dramatic change for Berezovsky as he had been for 13 years an enemy of the Kremlin. In a report which deepened the mystery, Russia’s Channel One state television claimed that Berezovsky had suffered several heart attacks in the past week.

Boris Berezovsky with Roman Abramovich in 2000 (AP)

Lord Bell, the PR executive who had acted as Berezovsky’s spokesman and was also a good friend, told The Sunday Telegraph that he had last seen the oligarch three weeks ago.

“He was extremely depressed,” said Lord Bell. “He has been very low since the court ruling against him. He had huge financial problems and personal problems, too.”

Berezovsky had sued Mr Abramovich over allegations that he had been intimidated into selling the football club owner shares in Sibneft, the Russian oil conglomerate. The ensuing legal battle is thought to be the most expensive of all time, with legal costs as high as £100 million.

In a devastating ruling against him in August last year, Mrs Justice Gloster had “found Mr Berezovsky an unimpressive, and inherently unreliable, witness, who regarded truth as a transitory, flexible concept, which could be moulded to suit his current purposes”.

Yelena Gorbunova and Boris Berezovsky (Reuters)

Lord Bell said yesterday that the ruling had wrecked his reputation and that he had never recovered. “That judge destroyed his morale. It destroyed his self-esteem,” he said.

Lord Bell added: “He was an extremely nice man, very kind to me and all the people around him. I had a huge amount of admiration for him.”

Berezovsky said after the trial: “Sometimes I have the impression that Putin himself wrote this judgment.”

Lord Bell said his friend, whom he had known for 14 years, had long lived in fear of his life after making enemies with Mr Putin.

“Russia is a very dangerous place,” said Lord Bell, the founder of Bell Pottinger, the public relations company.

The alarm over Berezovsky was raised at one of his homes in Ascot by his bodyguard. His private lawyer told Lord Bell that an ambulance had been called but that Berezovsky could not be revived.

Boris Berezovsky walks to his car after a press conference in Westminster in July 2007 (PA)

A South Central Ambulance Service spokesman said: “We were called at 3.18pm by a caller who was concerned for the welfare of a gentleman at an address in Ascot. A 67-year-old man was confirmed dead at the scene. The death is unexplained and is being investigated by Thames Valley Police.”

Berezovsky had been facing a new legal battle brought by his former girlfriend, with whom he had two children.

Elena Gorbunova had sat at Berezovsky’s side throughout his High Court battle with Mr Abramovich.

But the couple subsequently split up and in January she had applied to the court to freeze £200 million of his assets to prevent the sale of property in Britain and France.

At a preliminary hearing Mr Justice Mann said: “On the evidence, Mr Berezovsky is a man under financial pressure.”

Berezovsky, a former mathematician who had made his fortune in oil, aviation and television in the immediate aftermath of the fall of Communism, fled Russia in 2000 and was put on the country’s official wanted list in 2001 on charges of fraud and money laundering.

The British Government granted him political asylum in 2003 and he used London as a base from which to launch critical attacks on Mr Putin and to call for his overthrow.

Among his circle of exiles was Mr Litvinenko, who lived in a house in north London which was owned by Berezovsky.

The oligarch had survived several assassination attempts while in Russia, including a bomb that decapitated his chauffeur. Demyan Kudryavtsev, a journalist and former associate of Berezovsky, yesterday wrote on Twitter: “There were no signs of violent death.”

Alexander Dobrovinsky, a lawyer whose partnership has offices in Park Lane — where Berezovsky was also based — told Russia’s Rossiya 24 channel that he had been called from London and told the oligarch had committed suicide.

“He was in a terrible, awful state recently. He had so many debts, he was practically ruined,” said the lawyer. Dmitry Peskov, Mr Putin’s spokesman, claimed that Berezovsky had been in contact with the president in recent weeks, asking for forgiveness.

Mr Peskov told Rossiya 24: “Berezovsky addressed Putin in a letter, written by him personally, in which he admitted he made a lot of mistakes and was asking for forgiveness and to help him to return to the motherland.”

Thames Valley Police said Berezovsky’s death was being treated as “unexplained” and had cordoned off the property while investigations were carried out.